1. Field of the Invention
The present invention generally relates to a liquid crystal display (LCD), and more particularly, to a liquid crystal display having a low driving voltage and a high driving velocity.
2. Description of the Prior Art
In a liquid crystal display, incident light will produce different polarization or refraction when the alignments of liquid crystal molecules are different to produce gorgeous images. Since an LCD has the advantages of being lightweight, having low energy consumption, and being free of radiation emission, the LCD is widely used in various portable products, such as notebooks, personal data assistants (PDA), video cameras, etc., and even has a great potential to replace the conventional CRT monitor.
However, a conventional twisted nematic (TN) liquid crystal display and a conventional super-twisted nematic (STN) liquid crystal display have a very narrow viewing angle due to the structure of liquid crystal molecules and characteristics of optics, leading to a lot of limitation in application. Therefore, LCD manufacturers are all devoted to developing an LCD having a new structure to provide a wide viewing angle. For example, an in-plane switching mode liquid crystal display (IPS-LCD) is disclosed in U.S. Pat. No. 6,111,627 to effectively improve the problem of the viewing angle in the conventional twisted nematic LCD.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a cross sectional schematic diagram of a conventional in-plane switching mode LCD 10. FIG. 2 is a corresponding top view of the conventional in-plane switching mode LCD 10. As shown in FIG. 1, the conventional IPS-LCD 10 comprises a first substrate 12, a second substrate 14 in parallel with and opposite to the first substrate 12, at least one first electrode 16 and at least one second electrode 18 disposed on an upper surface of the second substrate 14, an isolation layer 15 disposed between the first electrode 16 and the second electrode 18 for preventing the first electrode 16 and the second electrode 18 from short circuiting, a first polarizer 13a and a second polarizer 13b disposed on a lower surface of the second substrate 14 and an upper surface of the first substrate 12 respectively, a first alignment layer 19a and a second alignment layer 19b disposed on the second substrate 14 and a lower surface of the first substrate 12 respectively, and a plurality of positive dielectric constant anisotropy liquid crystal molecules 17 filled in between the first substrate 12 and the second substrate 14.
The first electrode 16 is a counter electrode or a common electrode, and the second electrode 18 is a pixel electrode. The rubbing axis of the first alignment layer 19a determines the original orientation of the liquid crystal molecules 17. The rubbing axis of the second alignment layer 19b is the same as the rubbing axis of the first alignment layer 19a. The polarized direction of the first polarizer 13a is the same as the rubbing axis of the first alignment layer 19a, and the polarized direction of the second polarizer 13b is perpendicular to the polarized direction of the first polarizer 13a. 
As shown in FIG. 2, the first electrode 16 and the second electrode 18 are both in a comb shape. The first electrode 16 comprises a plurality of equally spaced branches 16a, 16b, 16c, which are in parallel with a signal line 22. The plurality of branches 16a, 16b, 16c are electrically connected to each other through a bar electrode 16x in parallel with a scan line 24, and the first electrode 16 is electrically connected to a common signal. The second electrode 18 and the first electrode 16 are in an interlaced arrangement. The second electrode 18 comprises a plurality of equally spaced branches 18a, 18b, which are in parallel with the signal line 22. The plurality of branches 18a, 18b are electrically connected to each other through a bar electrode 18x in parallel with the scan line 24. The bar electrode 18x is electrically connected to a thin film transistor 26 in a crossover region of the signal line 22 and the scan line 24 to control the turning on of a pixel unit of the IPS-LCD 10.
When the thin film transistor 26 is turned off, no voltage is applied between the first electrode 16 and the second electrode 18 and no electric field is formed. At this time, the longitudinal axis of the liquid crystal molecules 17 is in parallel with the rubbing axis of the first alignment layer 19a and the second alignment layer 19b. That means, the longitudinal axis of the liquid crystal molecules 17 is aligned to the direction that is coincident with the polarized direction of the first polarizer 13a. Therefore, no light can pass through the second polarizer 13b, and the observer cannot see any light emitted from the IPS-LCD 10. As a result, a perfect dark state of the IPS-LCD 10 is formed. When the thin film transistor 26 is turned on, the longitudinal axis of the liquid crystal molecules 17, affected by the electric field, gradually rotates from the original alignment direction to the alignment direction that is parallel to the electric field. That means, an angle difference is formed between the longitudinal axis of the liquid crystal molecules 17 and the polarized direction of the first polarizer 13a to allow light pass through, leading to a bright state of the IPS-LOD 10.
The conventional in-plane switching mode LCD can improve the problem of narrow viewing angle, which usually brings limitation to the conventional twisted nematic LCD. However, an LCD only having the advantage of wide viewing angle is not sufficient for today's requirement. When a voltage is applied between the pixel electrode and the counter electrode of the conventional in-plane switching mode LCD to generate a corresponding electric field, the electric lines nearby the color filter of the top substrate will bend. Thus, the rotation of the liquid crystal molecules is not as expected to affect various performance of the LCD.
It is therefore very important to resolve the above-mentioned problem when developing a wide viewing angle LCD to reduce the driving voltage and power consumption, improve the driving velocity to fulfill the requirement of animation projecting, increase the light efficiency of LCD and reduce cost of backlight, and simplify the processing and reduce total cost so as to produce a more competitive LCD product.